1. Field of the Invention
The present invention relates to a process for producing the salt of an alpha-sulfo fatty acid alkyl ester. More specifically, it relates to a process for producing the salt of an alpha-sulfo fatty acid ester from a fatty acid alkyl ester having an ester-bonded alkyl group having 2 to 8 carbon atoms at a high yield.
2. Description of the Prior Art
The salts of alpha-sulfo fatty acid alkyl esters can be generally produced by sulfonating starting fatty acid alkyl esters followed by neutralization of the resultant sulfonated products. However, since cleavage of the ester linkages or discoloration of the reaction products tends to occur during the sulfonation of fatty acid alkyl esters, several attempts have been made to prevent or suppress the cleavage of the ester linkages or discoloration of the reaction products. For example, U.S. Pat. No. 3,251,868 discloses a sulfonation process in which the starting ester is first sulfonated at a temperature of less than 70.degree. C. by dissolving therein the total of SO.sub.3 necessary to sulfonate the starting ester to such an extent that approximately half the amount of the ester is sulfonated and, then, the sulfonation is completed at a temperature of 70.degree. C. or more. Japanese Unexamined Patent Publication (Kokai) No. 57-126895 discloses a sulfonation process in which the starting ester is previously purified, to remove impurities contained therein, before the sulfonation. Furthermore, British Patent Specification No. 1145101 discloses a sulfonation process in which the starting ester is sulfonated at a temperature of 100.degree. C. to 140.degree. C. by downwardly flowing the starting ester in the form of a film. Japanese Unexamined Patent Publication (Kokai) No. 53-2419 discloses a sulfonation process in which the starting ester is first sulfonated and the reaction mixture is aged after cooling.
These proposed processes yield the salts of alpha-sulfo fatty acid methyl esters useful as a surfactant at a relatively high yield by neutralizing the resultant sulfonated products, as long as methyl esters are used as the starting fatty acid alkyl esters. However, when fatty acid alkyl esters having an alkyl group with 2 or more carbon atoms ester-bonded to the fatty acid are used as the starting material, the yield of the salts of the alpha-sulfo fatty acid alkyl esters obtained from the neutralization of the sulfonated products of the above-mentioned conventional processes is very low. Since the salts of alpha-sulfo fatty acid alkyl esters having an alkyl group with 2 or more carbon atoms have a surface active function (or detergency) identical to or greater than that of the salts of alpha-sulfo fatty acid methyl esters, the salts of the alpha-sulfo fatty acid alkyl esters having an alkyl group with 2 or more carbon atoms are useful surfactants comparable to the salts of alpha-sulfo fatty acid methyl esters. Thus, it is desirable to obtain the salts of alpha-sulfo fatty acid alkyl esters having an alkyl group with 2 or more carbon atoms at a high yield.